ˌwhite ˈelephant BrE AmE noun [countable]
[ Date: 1800-1900 ; Origin: From the supposed practice of the King of Siam, who gave to people he did not like a white elephant, which cost a very large amount of money to keep ]
something that is completely useless, although it may have cost a lot of money:
When the theatre first opened it was widely regarded as a white elephant.